1. Field of the Invention
The present invention generally pertains to the formation of optical lenses. Glass and/or plastic optical lenses are ground and cut from a limited number of standardized lens blanks. Depending upon an individual's prescription, each lens blank is ground and cut so that the optical qualities of the finished lens match the prescription required. By grinding, cutting and polishing, a limited number of lens blanks will produce large number of finished lenses.
In general, a lens is ground in a machine called a generator. The lens is held in the generator by a block that is attached to the lens via an adhesive substance. The lens block itself comes in various sizes with various inside curvatures to grind different lens materials (plastic or glass) to specified prescriptions.
More specifically, the present invention pertains to a unitary lens block for use in securing an optical lens during grinding, cutting and polishing of the lens by adhesively bonding a lens blank to the lens block.
2. Description of the Prior Art
When a lens blank is secured to a lens block and the block flexes during cutting, grinding or polishing of the lens, unwanted curves will be formed in the lens that make the lens optically worthless. Therefore, lens blocks must be made from rigid, non-flexing materials. U.S. Pat. No. Re. 31,897 describes a lens block that can be made from various rigid materials such as brass, steel, aluminum, zinc alloys and plastic.
Various problems have been encountered in the prior art when a metal lens block is utilized. First, a solder-like metal alloy has heretobefore been required to be used with metal blocks as the adhesive material. Such metal alloys are composed of heavy metals in different percentages. A typical composition example of such a metal alloy is: 45% bismuth, 23% lead, 19% indium, 8% tin and 5% cadmium. The different metals and percentages thereof give the alloy different melting points which become significant when holding either plastic or glass lens blanks. Higher melt point alloys are generally used to hold the heaver and harder to grind glass lens blanks. This high melt alloy, however, cannot be used with light plastic lenses since the higher melting point generally distorts the plastic lens. In addition, the composition of the metal alloy itself creates a problem. The Environmental Protection Agency has mandated that various of these materials be removed from federal optical labs by the middle of 1992 due to the toxicity of these metals.
Due to the problems associated with such metal alloy adhesive compositions, it has recently been proposed to utilize a wax as a replacement. Unfortunately, using a wax adhesive material does not work well with known metal blocks since the wax medium does not adhere well to the inside surface of the metal block.
The use of a plastic lens block, however, can be used with both alloy blocking and wax blocking if the plastic block remains rigid and if the blocking medium, alloy or wax, adheres to the block. Inherently, however, plastic blocks are less rigid than the prior known metal blocks and therefore have a greater tendency to flex during the cutting, grinding or polishing of a lens blank. Also, plastic blocks inherently adhere less well to wax or alloy.
Accordingly, there exists a need in the art for a plastic lens block that will remain structurally rigid during cutting, grinding or polishing of lens blanks such that the blocks will not flex and cause flaws in the optical lenses, and to which known blocking mediums will adhere properly and release from properly.